I am still quite noobish to the world of Nethack, but have been able to make it to this level once or twice while playing. Once I got in though, the level-drainer type enemies just had their way with me (vampires, wraiths, etc).

I have seen FAQs that say to just simply try to run from one end to the other, but by that time I'm back down to a level that Vlad the Impaler would laugh at.

2 Answers
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One thing that is very important in hell is fire resistance. The fire traps in Gehennom are particularly nasty because, unlike normal fire traps, they decrease your maximum HP unless you're resistant. You should also fireproof your armor (by reading a scroll of enchant armor while confused for example) and put all items that could burn or boil into a bag, so they won't be damaged or destroyed.

As to the level drainers: The best defense against level drain attacks is magic cancellation. If you have an item which offers level 3 magic cancellation, this will cause drain attacks to fail in most cases, making level drainers much more harmless.

For fighters it's also a good idea to have a silver saber as those are especially effective against demons and the undead (well, most undead - zombies don't care, but zombies aren't really dangerous at this stage anyway).

Also note that you can eat wraith corpses to gain levels.

General things you should do before you enter the end game:

Get your naked AC down to 2 or lower with divine protection (you can get it down to 0 quite easily via donations).

Have magic resistance and reflection.

Be fast (or preferably very fast)

Make sure all of your stats (except charisma) are close to or at their maximum

Enchant most of your armor to +4 or +5.

If you're a fighter, enchant your weapon to +6 or +7.

If you're a caster make sure that all your important spells are recent in your memory or you have the spellbook in your bag of holding. You do not want to forget magic missile while battling The Wizard or Death.

Make sure you have a means to detect portals (not important in Hell, but once you leave the dungeons, that's gonna come in handy and you do not want to have to go searching for gold detection scrolls after you have woken The Wizard).

You should also have a reliable way to cure sickness (blessed unicorn horn or cure sickness spell) as there will be a couple of enemies who can make you terminally ill.

Since the levels of Gehennom are usually mazes, the path to the stairs to the next level can become quite long. When you are ascending, this can be a problem because you are hunted by the Wizard Of Yendor. My advice is to prepare for the final run by cutting straight passages between stairs with the pick-axe. It takes time, of course, but it pays off when you have the Wizard on your tail.

Another way of getting to the stairs up fast is using teleport. However, spells cost more power than usual when carrying Amulet of Yendor and the Amulet also has an ability to hinder your teleportation. That's why cutting passages is a more reliable way.